In Ising spin glasses, the spin-glass transition temperature in the \(H-T\)  plane obeys de Almeida Thouless (AT) line [1]}. In Heisenberg spin-glass systems with weak random anisotropy, in the presence of the external magnetic field, the transverse and the longitudinal components of the spins may behave differently [2]}, [3]} . When the system is cooled at a constant magnetic field, the transverse spin component undergoes a spin freezing temperature which is called as Gabay-Toulouse (G-T) critical line [4]}. On the other hand, at a further lower temperature, there is a cross-over line where the longitudinal spin component freezes. This lower temperature cross-over line follows similar power law dependence as AT line in the \(H-T\)  plane [3]}, [6]}. It is indeed interesting to note here that two kinds of irreversible regimes - weak and strong - have been observed experimentally in concentrated or cluster spin-glass systems. For example upon lowering the temperature at the constant magnetic field ( \(H >\)  500 Oe) in a concentrated spin-glass system Cu\(_{0.94}\) Mn\(_{0.06}\) , one first encounter a weak irreversibility regime at a characteristic temperature, which represents a true phase transition with transverse freezing. This weak irreversibility follows the G-T relation [4]} with a coefficient close to the mean-field value for a Heisenberg system [3]}. Then at a further lower temperature, there is an onset of strong irreversibility, which is found to agree quantitatively with the mean-field d'Almeida-Thouless-like H-T crossover relation for longitudinal freezing in a Heisenberg system [3]}. In AuFe(5%) spin-glass alloy, using torque and magnetization measurements, the transverse component of the spins has been found to freeze at a higher temperature than the longitudinal spin component [10]}
